Switching power supplies that provide a regulated output generally require that the output voltage be sampled and fed back to a control circuit. When specifications require that the output be isolated from the input, present techniques provide two basic alternatives. The first technique is to power the control circuit by the input which requires the sampled output to be fed back through isolating devices such as optocouplers. Since optocouplers have generally poor DC transfer characteristics and very high temperature coefficients, output regulation for this method is often unsatisfactory. The second technique is to power the control circuit from an isolated auxiliary power source referenced to the output with switching components being driven by isolating driver transformers. While regulation achieved through this method may be satisfactory, circuit parts counts and weight can be greatly increased with the use of such circuits.
Isolated switching regulators have previously been disclosed. For example, U.S. Pat. No. 3,660,749 issued to Kadri on May 2, 1972 discloses an isolated DC to DC converter using pulse width modulation. Voltage regulation of the DC to DC converter is accomplished by modulating the pulse width of a driving signal applied to the switching devices. This modulation is synchronized with the switching action of the converter's inverter. For the push-pull type inverters used in the Kadri invention, the modulation control operates in response to successive half cycles of the inverter output having opposite signal polarities.
Although isolated, the feedback method employed in the Kadri patent utilizes flux variation as the feedback method which presents temperature compensation problems. The forward power transfer path has a temperature coefficient path that differs from the temperature path of the feedback path. These differing temperature coefficients result in substantial drift problems over wide temperature variations such as are encountered in military applications.
In view of the above problems with the prior art, there exists a need for isolated feedback circuit having a feedback path with a temperature coefficient equal and opposite to that of the forward power transfer path.